In a cellular communication system, a communication link via an RF channel is established between a mobile station, or subscriber, and a source base station. As a mobile station moves out of range of the source base station, the signal quality will degrade until the communication link would ultimately be broken, or the call "dropped". To avoid loss of the communication link resulting from a dropped call, the communication link is shifted from the source base station to a target base station. This process of making the shift is commonly referred to in the cellular communication area as a handover process.
A handover can be defined as a change of channel during a call, either because of degradation of the quality of the RF channel which includes, power level or communication link quality below a certain threshold, or because of the availability of another channel which can allow communication at a lower transmit power, or to prevent a mobile station from grossly exceeding the planned base station boundaries. A handover may occur during a call in progress (e.g. from a traffic channel to a traffic channel), or during the initial signaling during call set-up. The handover may be either from a channel on the source base site to another channel on a target base site or between channels on the source base site.
In digital cellular systems--such as time division multiple access (TDMA) and code division multiple access (CDMA) systems, a mobile assisted handoff (MAHO) process is utilized. In MAHO, the mobile station is provided with a list of candidate base stations. At intermittent times, the mobile will measure a signal quality parameter of transmissions from the listed base stations. The quality parameter may be signal strength, or an other appropriate parameter such as energy per chip per total noise (EC/Io), bit error rate (BER), frame erasure rate (FER), or color code. These measurements are gathered to determine a preferred list of target base stations to which handover may be directed.
Despite handover capability, communication link failure followed by a dropped call, may happen when the signal-to-noise (S/N) level drops quickly in either a link from the mobile unit to the base station, or vice versa, resulting in the mobile unit's inability to detect handover commands. In wireless communication systems, dropped calls adversely affect overall system performance. Dropped calls cause irritation to the end customers and a loss of revenue to the wireless communication system operator. Microcellular system configurations may be particularly susceptible to dropped calls since increased reuse of traffic channel frequencies contributes to a higher level of co-channel and adjacent channel interference. In many dropped call cases, the drop could be avoided if base stations that serve the call could maintain communication with the mobile unit long enough to deliver handover instructions which could instruct the mobile unit to tune to a new channel at a target base station. Once the source base station controller realizes that the communication is in danger of being lost, however, it is often too late to deliver handover instructions, with associated handover parameters, to the mobile unit.
Therefore, a need exists for an improved method for transferring a communication signal from a first fixed communication unit, or source base station, to a second fixed communication unit, or target base station, in a wireless communication system.